A photovoltaic (PV) cell is a semiconductor transducer for converting visible (about 400-700 nm) or near infrared (about 750-1000 nm) wavelengths of the electromagnetic spectrum into electrical energy. Photovoltaic cells are used in terrestrial and space power arrays to generate electric power. Lightweight space power arrays are in increasing demand as commercial, global telecommunication systems involving a constellation of satellites in low, medium, or geosynchronous orbits are necessary for relaying the RF communication signals. Reducing weight and cost for these relay satellites is important to the overall commercial success of these ventures.
Concentrator photovoltaic cells or arrays use focusing optics to concentrate or intensify incident solar radiation from a strength of one sun to many suns, i.e., on the order of 50-1,000 or more suns. Because of the concentration, the required active area of the cells is reduced. Concentrator cells are especially useful for space applications where the absolute mass of the array and its specific power (i.e., power per unit mass) is of greater concern. Typical Boeing concentrator solar cells and modules are described in U.S. Pat. Nos. 5,217,539; 5,123,968; 5,118,361; and 5,096,505, which I incorporate by reference.
Concentrator photovoltaic cells require precise alignment of the optical assembly with the sun (i.e., the incident radiation). Otherwise, the conversion efficiency suffers because the concentrating lens or lenses cause the light to miss the PV cell. For example, for a concentrator cell using Entech's silicone Fresnel concentrating lens (see U.S. Pat. No. 5,096,505), the cell generates power efficiently only within a range of.+-.2.degree. off absolute alignment with the incident radiation. If misalignment exceeds 4.degree., power generation drops essentially to zero. Between 2.degree. and 4.degree. of offset, the conversion efficiency declines markedly. The array is designed to have the minimal mass to supply the necessary power. So a decline in conversion efficiency translates to loss of mission capability. Therefore, with these systems, accurate tracking is crucial. Controlling the alignment requires a sophisticated tracking system to realign the cell or array as the relationship of the sun and satellite changes because of the earth's rotation and the orbiting of the satellite. Allowing a wider tracking angle through use of a concentrating coverglass will provide a significant advantage over conventional concentrator arrays.
To relax the tracking angle, one prior art system used a large secondary lens between a primary lens (like that of U.S. Pat. No. 5,096,505) and the cell. This secondary lens included a domed upper surface and inclined sidewalls. The secondary lens relied upon internal reflection at its sidewalls to redirect radiation toward the active area of the cell. These secondary lenses impose, in space applications, a mass penalty that compromises the benefit gained from their use.
Entech developed a prismatic cover slide to reduce or to eliminate wire grid reflection losses. The prismatic cover slide redirects incident light rays away from the wire grid lines and into the cell active area, thereby increasing the conversion efficiency. Without the prismatic cover slide, the incident radiation that hit the grid lines would be absorbed or reflected instead of being connected to electrical energy in the PV cell. Further details concerning this prismatic cover slide are provided in U.S. Pat. No. 5,217,539.
Lightweight concentrating coverglasses that would relax the tracking angle tolerance while protecting the cells from radiation damage would allow significant reductions in the necessary active area of the PV cells and in the overall and mass of space power arrays. Such a concentrating coverglass would combine the functions of a lens with a coverglass. Its use provides the high specific powers necessary for satellites.
A Fraas-type photovoltaic concentrator (FIG. 1) includes a molded silicone primary lens 20 having a substantially convex body 21 and four corners 23. Each corner 23 of the lens 20 is attached to the top of a strut or post 22 secured to a housing or panel 24 for the photovoltaic cell 25. Conventional electrical connections interconnect individual photovoltaic cells 25 into a power circuit of the desired current voltage and power. The distance that the primary lens, typically a domed Fresnel lens, is suspended above the housing 24 (i.e. the height of the post 22) depends on the focal length of the primary lens 20. The photovoltaic cell 25 may be any photovoltaic transducer capable of withstanding the heat associated with solar concentration, including silicon, single-crystal silicon, GaAs, GaInP, CuInSe.sub.2, or the like or tandem cells like Boeing's GaAs/GaSb or GaAs/CuInSe.sub.2 cells. Single junction or multi-junction photovoltaic cells can be used.
The housing 24 and electrical connections must be capable of withstanding the harsh conditions of space. Suitable materials are described, for example, in U.S. Pat. Nos. 5,096,505 or 5,021,099, which I incorporate by reference.
As described in U.S. patent application Ser. No. 08/468,811, the coverglass allows relaxation of the tracking angle. I compared the efficiency of a Fraas-type concentrator having both a primary lens and a concentrating coverglass of the present invention like that arrangement shown in FIG. 1 with the efficiency of a test concentrator photovoltaic cell having only a primary lens. I used a measuring system using a movable mirror mounted to the arm of a computer-controlled robot to direct collimated light onto the photovoltaic cell being tested at a predetermined angle of incidence. The photovoltaic cell was a tandem GaAs/GaSb cell with an upper cell having an active area of 0.245 cm.sup.2 and no cover slide. The primary lens was a domed Fresnel lens of 50.times. magnification having a focal length of 42 millimeters, measured from the concave surface of the peak of the dome. The test concentrator did not include a cover slide. The test concentrator was located directly under the light reflected from the movable mirror. The source of collimated light was a xenon flash lamp emitting light having a spectrum that approximates the solar spectrum. The position for normal incidence was established by removing the primary lens and replacing it with a mirror. Normal incidence was identified by the reflection of light from the mirror directly back to the source. Using a micrometer adjustment, the primary lens was positioned 42 mm above the upper surface of the coverglass or the upper surface of the GaAs in the case of the test cell. The surface of the photovoltaic cell and the tangent plane at the center of the dome lens were perpendicular to the optical axis. With the primary lens removed, the reference cell were exposed to collimated light. The GaAs cell was connected to 10 ohm resistors, where I measured and recorded the induced voltages. These data values are V.sub.1 and V.sub.ref1. I replaced the 10 ohm resistors with 0.1 ohm resistors, and the measured and recorded voltages across the resistors. These data values are V.sub.2 and V.sub.ref2.
The optical system efficiency (.eta.) is: ##EQU1## wherein: V.sub.1 =voltage without a primary lens flowing through a 10 ohm resistor.
V.sub.2 =voltage with a primary lens flowing through a 0.1 ohm resistor. PA1 R.sub.1 =10 ohms PA1 R.sub.2 =0.1 ohms. PA1 A.sub.cell =active area of the tandem solar cell, or 0.245 cm.sup.2 PA1 A.sub.lens =area covered by the lens, or 13.69 cm.sup.2 PA1 V.sub.ref1 =voltage from the reference cell a GaAs cell without a coverglass flowing through a 10 ohm resistor, measured simultaneously with V.sub.1. PA1 V.sub.ref2 =voltage from the reference cell flowing through a 10 ohm resistor, measured simultaneously with V.sub.2.
With the primary lens in place, corresponding data was recorded and the efficiency calculated for a variety of angles of incidence. For V.sub.1 and V.sub.ref1, the values obtained at normal incidence are used in all subsequent calculations. A robot moved the mirror automatically and data was recorded under computer control.
Identical steps were followed for a primary lens/concentrating coverglass combination. To compare the angular performance of both systems, each set of data was normalized, by dividing each efficiency value of each set by the maximum efficiency value for that set. For both sets of data, this operation will give a value of 1 at or near the normal incidence position. The two normalized plots are superimposed on FIG. 4.
Efficiency for the test concentrator begins to fall rapidly to zero at.+-.2.degree.. In contrast, the concentrating coverglass of the present invention allows the cell to continue at a higher level of conversion efficiency out to about.+-.4.degree., where it begins to fall off. Thus, by employing a secondary concentrator formed in accordance with the present invention, the tracking angle of a concentrator can be relaxed significantly (doubled).
A design tradeoff occurs between the impact on the total system performance from the added weight of the tracking system versus simply providing an array with a larger total active area of PV cells. That is, the decline in conversion efficiency can be compensated for either by adding a tracking control system, or by simply increasing the total area of the cells to accommodate the decline in efficiency for the worst case misalignment, or by a combination of these two options.
The present invention provides another option for space power systems. The design is an array of modest concentration with broadened tolerance in the tracking angle.